Cavern Dreamer is a mage minion card, from the Arena unique set.
Cavern Dreamer is unique to the Arena, only available to The Taverns of Time event and can only be obtained via drafting it during an Arena game.
Cavern Dreamer is uncraftable and cannot be crafted or be obtained from card packs.
Cavern Dreamer is a very strong early-game play. Its respectable 3 health combined with the lack of removal in Arena means that an early Dreamer can easily stay alive to generate value. It can potentially give early removal such as Arcane Shot, Frostbolt, and Shadow Word: Pain, buffs such as Blessing of Might and Earthen Might, or minion generation cards like Mirror Image and Lost in the Jungle; in any case, it is likely to provide a significant tempo advantage.